1. Field of the Invention
The present invention relates to methods and agents for modifying aqueous fluids and, in particular, to modifying the rheological behavior of aqueous fluids used in well drilling and similar subterranean operations.
2. Background of the Art
Fluids are used in well drilling operations, for example, to cool and lubricate the drilling bit, to carry away drilled solids and other debris, to suspend drilled cuttings and other debris when the fluid column is static, to control subsurface pressure, to prevent squeezing or caving of formations, to suspend propping agents, and to minimize damage to any potential production zone. In hydraulic operations fluids are used to transfer pressure from one location to another.
Drilling fluids and hydraulic fluids can be water based or oil based. Typically, water based drilling and hydraulic fluids can include one or more aqueous fluid thickening agents, lubricants, and corrosion inhibitors. The aqueous fluid can be fresh water or brine, and can include aqueous solutions of acids, alkali, lower alkanols (methanol, ethanol, and propanol), glycols, and the like, provided that the water miscible liquid does not adversely affect the viscoelastic properties of the aqueous fluid. Also included are emulsions of immiscible liquids in water and aqueous slurries of solid particulates such as clay.
Viscoelasticity is a desirable rheological feature in drilling and hydraulic fluids which can be provided by fluid modifying agents such as polymeric agents and surfactant gelling agents. Viscoelastic fluids are those in which the application of stress gives rise to a strain that approaches its equilibrium value relatively slowly. Viscoelasticity manifests itself in shear rate thinning behavior. For example, when such a fluid is passed through a pump or is in the vicinity of a rotating drill bit, the fluid exhibits low viscosity. When the shearing force is abated the fluid returns to its more viscous condition.
Another function of fluid modifying agents in oil drilling applications is permeability modification. Secondary recovery of oil from reservoirs involves supplementing by artificial means the natural energy inherent in the reservoir to recover the oil. For example when the oil is stored in porous rock it is often recovered by driving pressurized fluid, such as brine, through one or more drill holes to force the oil to a well bore from which it can be recovered. However, rock often has areas of high and low permeability. The brine will finger its way through the high permeability areas leaving oil in the low permeability areas.
Various methods have been employed to solve this problem. For example, U.S. Pat. No. 5,101,903 discloses a method for reducing the permeability of the more permeable zone of an underground formation having nonuniform permeability. The method comprises injecting into the formation a blend of surfactant and an alcohol, the blend being introduced in an amount effective to reduce the permeability of the more permeable zone of the formation. The preferred surfactant is an amine oxide such as dimethyltallowamine oxide delivered in water. A disclosed alcohol is isopropanol. The method may include the further step of injecting an alcohol slug following injection of the surfactant and alcohol blend.
U.S. Pat. No. 4,745,976 discloses a method for partially or completely blocking the high permeability regions of a reservoir. The technique is based upon the ability to induce phase changes in surfactant solutions by changing counterions or by adding small quantities of different surfactants. An aqueous solution of an ionic surfactant may have a viscosity only slightly different from brine but an increase in the salt concentration or addition of a multivalent counterion can cause the surfactant to form a solid precipitate or form a gel-like structure of high viscosity. In the method of U.S. Pat. No. 4,745,976, a first surfactant solution is injected into the formation followed by a water-soluble spacer fluid followed by a second surfactant solution. In situ mixing of the two surfactant solutions is affected by the tendency of different surfactant types to travel at different velocities through the reservoir. The compositions of the first and second surfactants solutions are chosen so that upon mixing, a precipitated or gel-like structure will form blocking the high permeability zone of the reservoir.